One-pot exchange reaction of acyl hydrazone combined with radical polymerization of styrene derivatives bearing acyl hydrazone moieties

Article abstract of DOI:10.1002/pola.29517

We developed a one-pot reaction combining an exchange reaction of the benzylidene moiety in an acyl hydrazone with styrene radical polymerization. The one-pot reaction of a styrene derivative bearing a 4-(dimethylamino)benzylidene acyl hydrazone moiety wa

Full text of DOI:10.1002/pola.29517

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One-pot exchange reaction of acyl hydrazone combined with radical
polymerization of styrene derivatives bearing acyl hydrazone moieties
Daisuke Nagai
,
1,2 Kazutoshi Kobayashi,2 Kento Nakamura,2 Takeshi Yamanobe2
1School of Food and Nutritional Science, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka-shi 422-8526, Japan
2Division of Molecular Science, Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515,
Japan
Correspondence to: D. Nagai (E-mail: daisukenagai@u-shizuoka-ken.ac.jp)
Received 1 August 2019; Revised 11 September 2019; accepted 23 September 2019
DOI: 10.1002/pola.29517
ABSTRACT: We developed a one-pot reaction combining an
exchange reaction of the benzylidene moiety in an acyl
hydrazone with styrene radical polymerization. The one-pot reac-
exchanged with 4-(methoxy)benzylidene acyl hydrazone, and
polymerization proceeded smoothly to provide the corresponding
polymer in 70% yield. Compared to traditional stepwise methods
and polymer post-polymerization modifications, this one-pot sys-
tem exhibits distinct advantages for the facile and efficient prepara-
tion of various functional polymers. © 2019 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2019
tion of
a styrene derivative bearing a 4-(dimethylamino)
benzylidene acyl hydrazone moiety was conducted in the pres-
ence of 4-cyanobenzaldehyde and pyridinium p-toluene sulfo-
nate/H2O for the exchange reaction and AIBN as the radical
polymerization initiator in N,N-dimethylformamide at 70 ꢀC for
20 h. The exchange reaction proceeded quantitatively, with the
4-(dimethylamino)benzylidene acyl hydrazone moiety being
KEYWORDS: Aldehydes; Polymers; Polymerization; Synthetic
methods
the one-pot combination of polymerization and various compati-
ble organic reactions to achieve polymers with the desired func-
tions is attracting more and more attention and is becoming an
important research topic for the development of polymerization
methodologies.
INTRODUCTION Performing multiple reactions simultaneously
or in tandem with polymerization is a new integrated synthetic
strategy to achieve polymers with unique functionalities.1–3 The
combination of different reactions in a one-pot system not only
avoids tedious intermediate purification steps but also provides
a powerful and exquisite strategy for sophisticated polymer syn-
thesis and modification.4–8 One-pot processes have been
achieved by the combination of selected compatible reactions
with polymerizations running in parallel, leading to polymers
with predesigned functionalities and structures.
Acyl hydrazones are stable under a wide range of conditions,
yet undergo covalent exchange reactions in the presence of
acid/water and aldehydes.15 Acyl hydrazones are hydrolyzed by
acid/water catalysts to yield hydrazide and aldehydes, and the
formed hydrazide can then react with another aldehyde to
afford a new acyl hydrazone exchanged at the benzylidene moi-
ety. This reaction can tolerate radical polymerization conditions.
Thus, we were encouraged to combine the exchange reaction of
acyl hydrazone with radical polymerization to develop new one-
pot methodologies for functional polymer syntheses.
For example, Sawamoto et al. combined metal-alkoxide-catalyzed
transesterification with ruthenium-catalyzed living radical polymer-
ization to efficiently produce sequence-regulated polymers in a
one-pot fashion.9,10 Haddleton et al. developed a novel one-pot
methodology through the combination of copper-catalyzed azide-
alkyne cycloaddition and atom transfer radical polymerization to
prepare polymers bearing various functional groups.6 Endo et al.
reported a one-pot reaction combining carbon dioxide fixation of
oxirane side-chain groups with radical polymerization.11 One-pot
strategies consisting of three-component reactions (Bignelli reac-
tion, Kabachnik–Fields reaction) and reversible addition–frag-
mentation chain transfer (RAFT) processes have also been reported
to afford polymers with various functional groups.12–14 These
successful efforts enrich and extend the content of polymer
chemistry. Due to these facile, versatile, and powerful features,
Our one-pot synthetic strategy involves an in situ acyl
hydrazone exchange reaction accompanied by radical poly-
merization as shown in Scheme 1.
EXPERIMENTAL
Materials and Instruments
4-Vinylbenzoic acid (Tokyo Kasei Kogyo, >97.0%), anhydrous
hydrazine (Tokyo Kasei Kogyo, >98.0%), and 4-dimethyl-
benzaldehyde (Tokyo Kasei Kogyo, >99.0%) were used as received.
© 2019 Wiley Periodicals, Inc.
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acyl hydrazone exchange reaction
organic layer was dried over sodium sulfate for 4 h and then
filtered. The filtrate was concentrated under reduced pres-
sure, and the residue was dried under vacuum to obtain
methyl 4-vinyl benzoate as a white powder in 89% yield.
n
O
H
, acid/H2O
O
NH
N
O
NH
N
O
NH
N
X
1H NMR (500 MHz, DMSO-d6) δ 3.84 (s, 3H), 5.43 (dd, 1H),
6.00 (dd, 1H), 6.82 (dd, 1H), 7.62 (d, 2H), 7.93 (d, 2H). 13C
NMR (500 MHz, DMSO-d6) δ 52.1, 117.4, 126.4, 128.8, 129.5,
135.7, 141.7, 166.0.
AIBN (3 mol%), solvent, 60 oC, 20 h
radical polymerization
N
X
N
exchangeable group
Synthesis of 4-Vinylbenzhydrazide
SCHEME 1 One-pot reaction combining exchange reaction of
acyl hydrazone with radical polymerization. [Color figure can be
viewed at wileyonlinelibrary.com]
Anhydrous hydrazine (0.762 mL, 28 mmol) was added
dropwise to a methanol (10 mL) solution containing methyl
4-vinylbenzoate (3.24 g, 20.0 mmol). The mixture was stirred
with refluxing for 17 h and concentrated under reduced pres-
sure. The residue was dissolved in dichloromethane
(40.0 mL), dried over sodium sulfate, and filtered. The filtrate
was concentrated under reduced pressure, and the residue
was dried under vacuum to obtain the mixture containing
4-vinylbenzhydrazide and methyl 4-vinylbenzoate (99:1),
which was used in the subsequent reaction.
4-Methoxybenzaldehyde (Tokyo Kasei Kogyo, >99.0%) was distilled
prior to use. 4-Cyanobenzaldehyde (Tokyo Kasei Kogyo, >98.0%)
and 4-nitrobenzaldehyde (Tokyo Kasei Kogyo, >95.0%) were rec-
rystallized from methanol. 2,20-Azobis(isobutyronitrile) (Tokyo
Kasei Kogyo, >98.0%) was used as received. Sulfuric acid (Wako
Pure Chemical, >95.0%), hydrochloric acid (Wako Pure Chemical,
>37.0%), and acetic acid (Kanto Chemical, >99.7%) were used as
received. Dichloromethane (Wako Pure Chemical, >99.5%), metha-
nol (Wako Pure Chemical, >99.8%), and sodium sulfate (Kanto
Chemical, >99.0%) were used as received. Trifluoroacetic acid
(Wako Pure Chemical, >99.5%) was distilled prior to use.
Pyridinium p-toluenesulfonate (PPTS, Tokyo Kasei Kogyo, >98.0%)
was recrystallized from methanol. N,N-Dimethylformamide (DMF,
Tokyo Kasei Kogyo, >99.5%) was dried over calcium hydride
(Aldrich, >95%) and distilled prior to use.
1H NMR (500 MHz, DMSO-d6) δ 4.49 (s, 2H), 5.34 (dd, 1H),
5.94 (dd, 1H), 6.77 (dd, 1H), 7.54 (d, 2H), 7.80 (d, 2H), 9.77
(s, 1H). 13C NMR (500 MHz, DMSO-d6) δ 117.4, 126.4, 128.8,
129.5, 135.7, 141.7, 162.2.
Synthesis of Styrene Derivative Bearing
4-(Dimethylamino)Benzilidene Acyl Hydrazone
4-Dimethylbenzaldehyde (0.46 g, 3.08 mmol) was added to a
solution of methanol (56 mL) and acetic acid (3.7 mL). After
stirring for 19 h at room temperature, the resulting mixture
was concentrated under reduced pressure and the residue
was dissolved in N,N-dimethylformamide (25.0 mL). The solu-
tion was added to distilled water (500 mL) and stirred for
3 h, and the precipitates were filtered. The precipitates were
recrystallized from tetrahydrofuran/hexane (v/v = 1/2) to
give the styrene derivative bearing a 4-(dimethylamino)
benzylidene acyl hydrazone moiety (SDA) as white crystals in
55% yield.
IR spectrum was recorded using a TheremoFisher Scientific
Nicolet iS50 spectrometer and the values were given in cm−1
.
1H and 13C NMR spectra were recorded on a Bruker Avance
III HD500 spectrometer using tetramethylsilane (TMS) as an
internal standard; the δ values are reported in parts per mil-
lion (ppm). Number-average (Mn) and weight-average (Mw)
molecular weights were estimated by size-exclusion chroma-
tography (SEC) using a system consisting of a Hitachi L-7100
pump, a Hitachi L-7400 refractive index (RI) detector, and
polystyrene gel columns (Tosoh TSK gels α-2500, α-3000, and
α-6000, with size exclusion limitations of 1 × 104, 1 × 105,
and 1 × 107, respectively). MALDI TOF mass spectrometric
measurements were performed using an AXIMA Performance
instrument (Shimadzu Co.) equipped with a nitrogen laser
(λ = 337 nm). The spectrometer was operated in linear and
reflection-positive ion modes with pulsed ion extraction. Sam-
ples were in tetrahydrofuran (100 mg mL−1) and 2 μL of ali-
quot of the mixture was deposited onto the sample target
plate.
1H NMR (500 MHz, DMSO-d6) δ 2.98 (s, 6H), 5.39 (dd, 1H),
5.96 (dd, 1H), 6.62–6.87 (m, 3H), 7.42–7.98 (m, 6H), 8.26 (s,
1H), 11.50 (s, 1H). 13C NMR (500 MHz, DMSO-d6) δ 40.0,
111.5, 116.0, 120.9, 126.3, 127.4, 129.3, 132.6, 135.8, 140.8,
149.2, 151.8, 163.3. MS (MALDI-TOF MS) m/z 293.47 (calcd
for M: 293.36).
One-Pot Reaction Combining Exchange Reaction of Acyl
Hydrazone with Radical Polymerization of a Styrene
Derivative Bearing an Acyl Hydrazone Moiety
Synthesis of Methyl 4-Vinylbenzoate
SDA (200 mg, 0.680 mmol), 2,20-azobis(isobutyronitrile)
(3.40 mg, 0.0207 mmol), 4-cyanobenzaldehyde (178 mg,
1.36 mmol), DMF (0.650 mL), distilled water (6.00 μL,
0.333 mmol), and PPTS (6.00 mg, 0.04 mmol) were fed into a
glass ampule. The ampule was cooled, degassed, sealed, and
heated at 70 ꢀC for 20 h. The mixture was poured into ethanol
(60 mL), and the resulting precipitate was collected by suction
Sulfuric acid (4 mL) was added dropwise to a methanol solu-
tion (160 mL) containing 4-vinylbenzoic acid (4.0 g, 27 mmol)
at 0 ꢀC. After addition, the mixture was stirred with refluxing
for 17 h and a 10% aqueous solution of sodium hydrogen car-
bonate (240 mL) was added for neutralization. The products
were extracted using dichloromethane (160 mL × 3). The
2
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filtration and dried under vacuum to obtain the corresponding
polymer in 70% yield as a white powder. SEC analysis (eluent:
DMF containing 0.01 M LiBr, polystyrene standards) of the
formed polymer was carried out to estimate the number-
average molecular weight (Mn) and the molecular weight dis-
tribution (Mw/Mn) (Mn = 6900, Mw/Mn = 2.09).
1H NMR (500 MHz, DMSO-d6) δ 0.78–2.12 (br, 3H), 2.90
(s, 0.01 × 6H), 6.25–8.21 (br, 8H), 8.42 (br, s, 1H), 11.58
(br, s, 1H).
RESULTS AND DISCUSSION
A styrene derivative bearing 4-(dimethylamino)benzylidene
acyl hydrazone (SDA) was synthesized in three steps
(Scheme 2). 4-Vinylbenzoic acid was reacted with methanol in
the presence of sulfuric acid to afford methyl 4-vinylbenzoate
in 74% yield. The reaction of methyl 4-vinylbenzoate with
anhydrous hydrazine in methanol under reflux for 18 h
afforded a 99:1 mixture of 4-vinylbenzhydrazide and methyl
4-vinylbenzoate. The mixture was then reacted with
4-dimethylaminobenzaldehyde in the presence of acetic acid.
Recrystallization of the resulting mixture from tetrahydrofu-
ran and hexane removed methyl 4-vinylbenzoate to provide
the styrene derivative bearing 4-(dimethylamino)benzylidene
acyl hydrazone (SDA). The 1H NMR spectra of SDA and the
FIGURE 1 1H NMR spectra (CDCl3, rt) of 4-vinylbenzhydrazide,
SDA, and 4-(dimethylamino)benzaldehyde. [Color figure can be
viewed at wileyonlinelibrary.com]
radical polymerization conditions. Primary amine groups of
hydrazides are prone to nucleophilically attack toward vinyl
groups of styrene derivatives. Our strategy to overcome this
problem was to transfer the equilibrium to the formation of
the new acyl hydrazone moiety by tuning the electronic prop-
erties of the aldehyde exchanged in the system. Thus, the
excellent tolerance of many functional groups during radical
polymerization allows for its combination with the exchange
reaction of acyl hydrazone.
1
starting materials are shown in Figure 1. In the H NMR spec-
trum of SDA, no signals corresponding to the two amino
groups of 4-vinylbenzhydrazide (E and F) and the aldehyde
group of 4-dimethylaminobenzaldehyde (P) were observed.
In order to carry out the one-pot exchange reaction of acyl
hydrazone/radical polymerization successfully, the exchange
reaction must not affect the progress of the radical polymeri-
zation. Radical polymerization of 4-vinylbenzaldehyde has
been reported to proceed in a controlled manner, showing
that aldehydes formed by hydrolysis are tolerant to the
The one-pot reaction of SDA was carried out in the presence
of 4-methoxybenzaldehyde and trifluoroacetic acid/H2O for
the exchange reaction and AIBN as the radical polymerization
initiator in DMF at 60 C for 20 h (Scheme 3).
4-Methoxybenzaldehyde was selected as the aromatic alde-
hyde to be exchanged, because the methoxy group is less elec-
tron donating than the dimethylamino group, thus leading to
higher electrophilicity of the carbonyl group. The nucleophilic
attack of hydrazide on 4-methoxybenzaldehyde predominates
that on 4-dimethylaminobenzaldehyde formed in situ. The
SCHEME 3 One-pot reaction combining exchange reaction of
acyl hydrazone with radical polymerization of SDA. [Color figure
can be viewed at wileyonlinelibrary.com]
SCHEME
2
Synthesis of styrene derivative bearing 4-
(dimethylamino)benzylidene acyl hydrazone.
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reaction to the synthesis of various functional polymers.
Therefore, utilization of the less acidic acetic acid (pKa = 4.76)
and pyridinium p-toluene sulfonate (PPTS, pKa = 5.21) for the
one-pot reaction was examined. The one-pot reaction in the
presence of acetic acid/H2O decreased the exchange efficiency
(entry 4), suggesting that the lower hydrophobicity reduced
the affinity for the organic reaction (i.e., the catalytic activity
in DMF). On the other hand, although the exchange reaction
proceeded quantitatively in the presence of PPTS/H2O, poly-
merization did not occur (entry 5). This is because the added
amount of powdery PPTS was very large (342 mg) relative to
that of SDA (200 mg), which created a heterogeneous system
in DMF (0.65 mL) that inhibited polymerization. In order to
promote both polymerization and the exchange reaction, the
effect of the concentration of PPTS was examined (Fig. 3).
When the concentration of PPTS was reduced from 0.24 M to
0.12 M, polymerization did not occur due to heterogeneity,
while the exchange reaction proceeded quantitatively. On the
other hand, when the PPTS concentration was reduced to
0.06 M, both polymerization and the exchange reaction
proceeded (42% yield of polymer and 99% exchange effi-
ciency). Further reductions in PPTS concentration to 0.03 and
0.015 M decreased the exchange efficiency to 98%, while the
polymerization yields were nearly constant. The lower con-
centrations of PPTS suppress the hydrolysis of the imine
groups of SDA and the formation of the new acyl hydrazone
moiety. These results indicate that the optimal concentration
of PPTS is 0.06 M.
FIGURE 2 1H NMR spectrum of the obtained polymer (DMSO-
d6, rt).
polymerization proceeded smoothly to afford the corresponding
polymer in 84% yield, whose Mn and Mw/Mn values were esti-
mated to be 12,300 and 1.90, respectively. The 1H NMR spec-
trum of the obtained polymer contained peaks assignable to the
methoxy protons, and the integrated intensity ratio of the
methoxy group to the dimethyl amino group was 84:16 (Fig. 2),
indicating that the 4-(dimethylamino)benzylidene acyl hydra-
zone moiety was exchanged with 4-(methoxy)benzylidene acyl
hydrazone with 84% efficiency (Table 1, entry 1).
To enhance the exchange efficiency of the side-chain reaction,
the effect of substituents on the aromatic aldehyde was exam-
ined. Instead of 4-methoxybenzaldehyde, the utilization of aro-
matic aldehydes bearing electron-withdrawing groups such as
cyano and nitro groups at the para position increased the
exchange efficiency (entries 2 and 3 in Table 1). This indicates
that electron-withdrawing groups increase the electrophilicity
of the carbonyl group on the aromatic aldehyde. This acceler-
ates the electrophilic attack on the carbonyl groups by the
hydrazides formed in situ, providing a higher exchange
efficiency.
In order to further enhance polymerization, the effects of
water and reaction temperature were examined (Table 2).
Because water is a poor solvent for polymerization, its con-
centration was reduced. Decreasing the water concentration
from 3.24 M to 1.63 and 0.81 M allowed polymerization to
occur with similar yields (38–42%, entries 1–3). However, the
exchange efficiency decreased, indicating that the reduction in
the water concentration suppressed the hydrolysis of the
imine groups of SDA and the formation of the new acyl
hydrazone moiety. Finally, the effect of temperature was
examined. Increasing the reaction temperature from 60 to
70 ꢀC increased the yield of the polymer (70%) while
maintaining the high exchange efficiency (99%, entry 4). How-
ever, further increasing the reaction temperature decreased
Because the acidity of trifluoroacetic acid is very high
(pKa = 0.23), a diversity of reaction conditions cannot be toler-
ated. Acidic hydrolysis restricts the application of the one-pot
TABLE 1 One-Pot Reaction Combining Exchange Reaction of Acyl Hydrazone with Radical Polymerization of SDAa
Entry
X
Acid
M
n (Mw/Mn)b
Yield/%c
X/SDAd
1
2
3
4
5
OMe
CN
TFA
12,300 (1.90)
9100 (1.18)
4900 (1.29)
13,800 (2.93)
–
84
43
27
85
–
84/16
97/3
TFA
NO2
CN
TFA
99/1
Acetic acid
PPTS
41/59
e
CN
–
a Conditions: [SDA]0 = 1.0 M; [aromatic aldehyde]0 = 2.0 M; [AIBN]0 = 3 mol
% to SDA; [acid]0 = 1.92 M; [H2O]0 = 0.5 M; reaction temperature = 60ꢀC;
reaction time = 20 h.
c Ethanol insoluble parts.
d Determined 1H NMR analysis.
e Monomer was exchanged quantitatively.
b Estimated by SEC analysis (eluent: N,N-dimethylformamide, polysty-
rene standards).
4
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TABLE 2 Effects of Water Concentration and Reaction Temperature in the One-Pot Reaction Combining Exchange Reaction of Acyl
Hydrazone with Radical Polymerization of SDA in the Presence of PPTS/H2O catalysta
Entry
H2O (M)
Temp (ꢀC)
M
n (Mw/Mn)b
Yield (%)c
X/SDAd
1
2
3
4
5
3.25
1.63
0.81
3.25
3.25
60
60
60
70
80
7700 (1.90)
8800 (1.85)
7300 (0.81)
6900 (2.09)
4500 (1.99)
42
40
36
70
33
>99/>1
98/2
98/2
99/1
97/3
a Conditions: [SDA]0
=
1.0 M; [4-cyanobenzaldehyde]0
=
2.0 M;
c Ethanol insoluble parts.
[AIBN]0 = 3 mol% to SDA; [PPTS]0 = 0.06 M; reaction time = 20 h.
d Determined 1H NMR analysis.
b Estimated by SEC analysis (eluent: N,N-dimethylformamide, polysty-
rene standards).
both the polymer yield (33%) and exchange efficiency (97%).
The high temperature disturbed the equilibrium, thus affecting
the formation of the new acyl hydrazone moiety. Therefore, the
optimal conditions in the one-pot reaction were determined to
100
100
100
100
9898
9696
80
80
be: [SDA]0
=
1.0 M; [4-cyanobenzaldehyde]0
=
2.0 M;
660 0
[AIBN]0 = 3.0 mol % to SDA; [PPTS]0 = 0.06 M; [H2O]0 = 0.5 M;
reaction temperature = 70 ꢀC; reaction time = 20 h.
9494
9292
40
40
Reaction monitoring under the optimal conditions revealed
that the exchange reaction for the acyl hydrazone moiety was
faster than polymerization (Fig. 4). The acid-catalyzed acyl
hydrazone exchange reaction proceeded with an efficiency of
45% within 3 min, at which point polymerization had only
achieved 34% conversion of SDA. The exchange reaction then
accelerated with the efficiency reaching 100% within 10 min,
while polymerization was at 36% conversion. Polymerization
continued gradually under these reaction conditions and
reached a plateau of 70% conversion at 10 h. These results
suggest that the one-pot reaction is essentially composed of
two stages. In the first stage, the exchange reaction of the
benzylidene moiety in the acyl hydrazone of SDA is fast due to
the lower steric hindrance, forming the new monomer, the
styrene derivative bearing the 4-(cyano)benzylidene acyl
hydrazone. The second stage is homopolymerization of the
new monomer, which is considerably slower.
20
20
90
0
90
0
0
0.3
0.2
0.1
concentration of PPTS (M)
FIGURE 3 Effect of concentration of PPTS in one-pot reaction
combining exchange reaction of acyl hydrazone with radical
polymerization
[4-cyanobenzaldehyde]0 = 2.0 M; [AIBN]0 = 3.0 mol % to SDA;
[H2O]0 = 0.5 M; reaction temperature = 60 ꢀC; reaction time = 20 h.
of
SDA.
Conditions:
[SDA]0
=
1.0 M;
100
100
80
80
60
40
CONCLUSIONS
60
A new horizon with regard to acyl hydrazone exchange reac-
tions in polymer chemistry is presented. Through a one-pot
strategy, the exchange reaction of the benzylidene moiety in
an acyl hydrazone was found to be compatible with radical
polymerization. The one-pot reaction of a styrene derivative
bearing a 4-(dimethylamino)benzylidene acyl hydrazone moi-
ety was conducted in the presence of 4-cyanobenzaldehyde
and pyridinium p-toluene sulfonate/H2O for the exchange
reaction and AIBN as the radical polymerization initiator in
DMF at 70 ꢀC for 20 h. Polymerization proceeded smoothly to
afford the polystyrene derivative bearing 4-(cyano)
benzylidene acyl hydrazone moieties in 70% yield and with
99% exchange efficiency. Compared to traditional stepwise
methods and polymer post-polymerization modifications, this
one-pot system exhibits distinct advantages for the facile and
40
20
0
20
0
0
5
10
15
20
25
reaction time (h)
FIGURE 4 Kinetic study of one-pot reaction combining exchange
reaction of acyl hydrazone with radical polymerization of SDA.
Conditions: [SDA]0 = 1.0 M; [4-cyanobenzaldehyde]0 = 2.0 M;
= 3.0 mol % to SDA; [H2O]0 = 0.5 M; reaction
temperature = 70 ꢀC; reaction time = 20 h.
[AIBN]0
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